Organic light emitting diode (OLED) which functions as a current type light emitting device has increasingly been applied to high performance active matrix organic light emitting diodes. Regarding a traditional passive matrix OLED, as its display size increases, a shorter driving time for a single pixel is needed and thus there is a need to increase the transient current, which increases the power consumption. Meanwhile, the application of large current may cause too large voltage drop in the ITO (Indium Tin Oxide) line, and cause the operating voltage of the OLED to be too high, and thus its efficiency would be lowered. Whereas regarding an AMOLED (Active Matrix OLED), the OLED current is input with a line by line scan of switch transistors, and these problems may be resolved.
In the design of the pixel circuit of the AMOLED, the main problem to be solved is the brightness nonuniformity of OLED devices driven by various AMOLED pixel driving units.
First, AMOLED uses TFTs (Thin Film Transistors) to construct a pixel driving circuit to provide corresponding driving current for the light emitting device. Generally, low temperature polycrystal silicon TFTs or Oxide TFTs are mostly used. Compared with a general amorphous silicon TFT, the low temperature polycrystal silicon TFT and the Oxide TFT have larger mobility and more stable characteristics, and are more suitable for be applied to AMOLED display. However, due to the limitation of crystallization process, the low temperature polycrystal silicon TFTs fabricated on a large area glass substrate usually have nonuniformity on electrical parameters such as threshold voltage, mobility and so on. Such nonuniformity will be converted to differences among the driving currents and brightnesses of the OLED devices which can be perceived by human eyes, i.e., a mura phenomenon. Although the Oxide TFT has good uniformity in terms of process, similar to the amorphous silicon TFT, its threshold voltage will drift when voltage and high temperature are applied for a long time. TFTs in different parts of the panel have different threshold drift amount due to different display pictures, which would cause display brightness difference. Because this difference is related to the picture displayed previously, it is usually presented as an afterimage phenomenon.
Since the light emitting device of the OLED is a current driven device, in the pixel driving unit for driving the light emitting device to emit light, the influence of the threshold characteristic of the driving transistor on the driving current and the final display brightness is great. Both the voltage stress and the illumination subjected by the driving transistor cause its threshold to drift, and such a threshold drift may be embodied as the brightness nonuniformity in the displaying effect.
In addition, in order to eliminate the influence caused by the threshold voltage difference of the driving transistors, in the pixel circuit of a common AMOLED, the structure of the pixel circuit is usually designed with higher complexity, which will result in the reduction of the fabrication yield for the pixel circuit of the AMOLED.
The present disclosure provides a pixel driving unit and a driving method thereof, and a pixel circuit.